U.S. patent number 5,070,230 [Application Number 07/501,867] was granted by the patent office on 1991-12-03 for electrically heatable windshield.
This patent grant is currently assigned to Asahi Glass Company Ltd.. Invention is credited to Akira Hirano, Hisashi Nishiyama, Koichi Osada.
United States Patent |
5,070,230 |
Osada , et al. |
December 3, 1991 |
Electrically heatable windshield
Abstract
An electrically heatable windshield comprises two glass plates
bonded to each other with an interlayer therebetween and a
transparent conductive layer formed at contacting surface between
one of the glass plates and the interlayer so that the windshield
is heated by supplying a current to the transparent conductive
layer through a bus bar for feeding power. A colored layer such as
a ceramic color print is formed at the peripheral portion of the
surface, in contact with the interlayer, of at least one of the
glass plates. The bus bar is formed on the colored layer without
expanding. A transparent protective layer is formed so as to cover
the edge portion of the colored layer which is near the center of
the glass plates, and the transparent conductive layer is formed
over the transparent protective layer.
Inventors: |
Osada; Koichi (Yokohama,
JP), Nishiyama; Hisashi (Yokohama, JP),
Hirano; Akira (Yokohama, JP) |
Assignee: |
Asahi Glass Company Ltd.
(Tokyo, JP)
|
Family
ID: |
26375307 |
Appl.
No.: |
07/501,867 |
Filed: |
March 29, 1990 |
Foreign Application Priority Data
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Mar 31, 1989 [JP] |
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36270[U] |
Oct 9, 1989 [JP] |
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117961[U] |
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Current U.S.
Class: |
219/203; 219/543;
338/309; 219/547; 52/171.2 |
Current CPC
Class: |
B32B
17/10761 (20130101); H05B 3/86 (20130101); B32B
17/10174 (20130101); B32B 17/10036 (20130101); H05B
2203/013 (20130101) |
Current International
Class: |
B32B
17/10 (20060101); B32B 17/06 (20060101); H05B
3/84 (20060101); H05B 3/86 (20060101); H05B
003/26 () |
Field of
Search: |
;219/203,522,543,547,541
;338/309,308,328,327 ;52/171 ;428/594,632,633 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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62456 |
|
Jan 1987 |
|
JP |
|
62457 |
|
Jan 1987 |
|
JP |
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62-99191 |
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Jun 1987 |
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JP |
|
Primary Examiner: Reynolds; Bruce A.
Assistant Examiner: Switzer; Michael D.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt
Claims
We claim:
1. An electrically heatable windshield comprising:
two glass plates bonded to each other with an interlayer
therebetween;
a transparent conductive layer formed at the contacting surface
between one of said two glass plates and the interlayer so that the
windshield is heated by supplying a current to the transparent
conductive layer through a bus bar for feeding power;
a colored layer formed at the peripheral portion of the surface, in
contact with the interlayer, of at least one of said two glass
plates, said bus bar being formed on said colored layer without
extending out from the edge of said colored layer; and
a transparent protective layer formed so as to cover the edge
portion of the colored layer which is nearest the center of the
glass plates, said transparent conductive layer being formed over
said transparent protective layer.
2. The electrically heatable windshield according to claim 1,
wherein said transparent protective layer is formed of a material
having a refractive index of 2.0 or lower.
3. The electrically heatable windshield according to claim 1 or 2,
wherein said transparent protective layer is a metal oxide film
which is formed by baking a liquid containing a material capable of
forming a metal oxide by baking.
4. The electrically heatable windshield according to claim 1 or 2,
wherein said transparent protective layer is a layer formed by the
solidification of molten glass frit.
5. The electrically heatable windshield according to claim 1,
wherein the thickness of said transparent protective layer is 5
.mu.m or less.
6. The electrically heatable windshield according to claim 1,
wherein said transparent protective layer is formed so as to cover
the portion of said colored layer where said transparent conductive
layer overlaps with said colored layer.
7. The electrically heatable windshield according to claim 1,
wherein said transparent protective layer is formed so as to cover
the portion of said colored layer and said bus bar where said
transparent conductive layer overlaps with said colored layer and
said bus bar except the contacting portion of said bus bar to said
transparent conductive layer.
8. The electrically heatable windshield according to claim 1,
wherein one end of said transparent protective layer is within a
gradation print of said colored layer.
9. The electrically heatable windshield according to claim 1,
wherein said colored layer is a ceramic color print.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrically heatable
windshield having an area heating element on the surface of the
glass plate for the purpose of eliminating moisture condensation,
melting ice or snow deposited thereon, and for defogging or
anti-fogging as well as for improving durability to a current
supplied to the area heating element.
2. Description of the Related Art
There have been known various types of electrically heatable
windshields having a transparent conductive layer as an area
heating element on the surface of the glass plate in order to
prevent moisture condensation or to melt ice or snow deposited
thereon.
FIG. 7 shows an example of a conventional technique. Namely, a
conventional laminated glass comprises an outer glass plate 50 and
an inner glass plate 52 bonded to each other through an interlayer
51 made of a material such as polyvinylbutyral, wherein a colored
layer 54 for shielding is formed at the peripheral portion of the
boundary surface between the outer glass plate 50 and the
interlayer 51 and a transparent conductive layer 53 as an area
heating element is formed at the boundary between them.
On the other hand, a bus bar 55 for supplying power is formed as a
laminated layer on the colored layer 54 so that power is supplied
to the transparent conductive layer 53 through the bus bar to
thereby heat the glass surface.
However, since the bus bar 55 was partially extended from the
colored layer 54 (i.e. an extending portion 56 was formed) as shown
in FIG. 7, the breaking of a coating layer often happened at the
boundary between the bus bar which was formed by printing and the
transparent conductive layer 53 when a current was supplied to the
bus bar.
Therefore, an improvement of the boundary has been studied.
Measures to minimize the breaking of layer have been concentrated
to the bus bar. For instance, there are proposals that an edge
portion of the bus bar is tapered (Japanese Unexamined Utility
Model Publication No. 457/1987), that the bus bar has a two-step
structure at the boundary (Japanese Unexamined Utility Model
Publication No. 456/1987) and that the bus bar is provided with a
protective print layer (Japanese Unexamined Utility Model
Publication No. 99191/1987).
Heretofore, the bus bar 55 was formed partly extended from the
colored layer 54 as shown in FIG. 7. The strength of adhesion
between the bus bar and the colored layer was not always
sufficient. Accordingly, there was a fairly large difference of
potential between them, and the breaking of layer often occurred
due to the potential difference.
Techniques described in the above-mentioned publications were,
therefore, proposed to improve the boundary between the bus bar 55
and the transparent conductive layer 53. Namely, these techniques
provided some progress in that the boundary between the bus bar 55
and the transparent conductive layer 53 was improved, and the
breaking of layer was reduced.
On the other hand, the fact that the bus bar is partially extended
from the colored layer means that the color of the bus bar is
visible from the outside of an automobile, this is against the
standard of safety ruled by the national regulation. In the social
circumstances, a demand of completely concealing the bus bar by the
colored layer has been increasing.
However, in the conventional techniques (including the
above-mentioned techniques even though there are some improvement),
consideration that the bus bar should be entirely located on the
colored layer has not been made, and it is still insufficient in
the performance of the electrically heatable windshield.
Namely, as shown in a Comparative Example (FIG. 10) in which a bus
bar is entirely on a colored layer in an electrically heatable
windshield, when a current is supplied to the electrically heatable
windshield after it has been left at a high temperature condition
(80.degree. C., 28 days), the breaking of the transparent
conductive layer was resulted at the contacting portion of the
colored layer to the transparent conductive layer, especially at
the end portion of the colored layer which is near the center of
the glass plate.
The inventors of this application examined the cause of breaking
and found that the following fact. There were particles of pigment
having a diameter of about 1 .mu.m on the colored layer after the
baking process. On the other hand, the film thickness of the
transparent conductive layer was thin as 0. 1 .mu.m. Accordingly,
the breaking of layer was resulted at the boundary between the bus
bar and the transparent conductive layer. In particular, the
breaking of layer was remarkable at the end portion of the colored
layer which is near the center of the glass plate.
As the colored layer, a colored layer formed by a ceramic color
print or formed by printing an organic series paint is used. The
ceramic color print is composed of the glass frit, pigment and one
or more additives. A bonding strength to glass mainly owes to the
glass frit because it can be molten.
When the glass frit is baked, it is molten to exhibit a flat
surface. Accordingly, the glass frit causes no problem. However,
when the particle diameter of a component other than the glass frit
is large or temperature of baking is insufficient, unevenness of
the surface of the ceramic color print or the colored layer is
large. Accordingly, irregularity in current conduction takes place
at the colored layer, or a resistance between terminals by bonding
the laminate glass becomes large. This creates the breaking of
layer at the boundary between the colored layer and the transparent
conductive layer.
SUMMARY OF THE INVENTION
It is therefore, an object of the present invention to prevent the
breaking of layer by covering an uneven surface resulted by a
component of pigment in the colored layer in an electrically
heatable windshield in which a bus bar is formed on the colored
layer.
It is an object of the present invention to provide an electrically
heatable windshield capable of improving the durability to a
current and having excellent performance and outer appearance.
In accordance with the present invention, there is provided an
electrically heatable windshield comprising two glass plates bonded
to each other with an interlayer therebetween and a transparent
conductive layer formed at the contacting surface between one of
the said glass plates and the interlayer so that the windshield is
heated by supplying a current to the transparent conductive layer
through a bus bar for feeding power, the electrically heatable
windshield being characterized in that a colored layer such as a
ceramic color print is formed at the peripheral portion of the
surface, in contact with the interlayer, of at least one of the
glass plates; the bus bar is formed on the colored layer without
extending out from the edge of the colored layer; a transparent
protective layer is formed so as to cover the edge portion of the
colored layer which is near the center of the glass plates, and the
transparent conductive layer is formed over said transparent
protective layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged longitudinal cross-sectional view partly
broken of an embodiment of the electrically heatable windshield for
an automobile according to the present invention;
FIG. 2 is a plane view of an outer glass plate;
FIG. 3 is a cross-sectional view taken along a line A--A' in FIG.
2;
FIG. 4 is a longitudinal cross-sectional view partly omitted of an
embodiment of the electrically heatable windshield of the present
invention formed in accordance with Examples 1 and 2;
FIG. 5 is a plane view partly omitted of an embodiment of the
electrically heatable windshield of the present invention in a case
that the gradation print of a colored layer is formed;
FIG. 6 is a cross-sectional view taken along a line C--C' in FIG.
5;
FIG. 7 is an enlarged longitudinal cross-sectional view partly
broken of a conventional electrically heatable windshield in which
a bus bar is partly extended from the colored layer;
FIG. 8 is a plane view of an outer glass plate in which the bus bar
is formed on a ceramic color print without expanding;
FIG. 9 is a cross sectional view taken along a line B-B' in FIG. 8:
and
FIG. 10 is a cross-sectional view partly omitted of an electrically
heatable windshield formed in accordance with Comparative
Example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be
described.
In the present invention, it is preferable to form a transparent
protective layer by a material having a refractive index of 2.0 or
less so that the transparent protective layer 16 is not conspicuous
in its outer appearance. Most preferably, the refractive index is
in a range of from 1.4-1.6 so as to be close to the refractive
index of glass which is used as a substrate, whereby optical
interference by the protective layer is suppressed and the
protective layer is not conspicuous in appearance.
The material having a refractive index of 2.0 or less may be, for
instance, a single composition or a mixture of a metal oxide such
as SnO.sub.2 (refractive index n=2.0), SiO.sub.2 (n=1.5), ZrO.sub.2
(n=2.0), In.sub.2 O.sub.3 .multidot.SnO.sub.2 (n=1.9), or a
composite oxide comprising an oxide having a high refractive index
such as TiO.sub.2 (n=2.3), Ta.sub.2 O.sub.5 (n=2.1) or the like and
the above-mentioned oxide having a low refractive index
incorporated at an appropriate proportion.
There are four methods to form the transparent protective layer 16
of the present invention.
In the first method, a mixture of glass frit and a thickening agent
is coated on a glass sheet by printing and baking the mixture.
In the second method, an organic metal salt such as acetylacetonate
or alkoxide, halide, acetate, nitrate or metal salt such as a
chelate compound, of the metal of the metal oxide which is to be
formed as the transparent protective layer 16 is dissolved in a
solvent such as alcohol, an aromatic compound such as benzene or
the like, or a chlorine series solvent, and thus obtained solution
(hereinbelow referred to as a metal salt solution) is coated on a
substrate. Then, the coated solution is thermally decomposed to
thereby form a film of metal oxide.
In the third method, a solution obtained by dissolving a metal
alkoxide of metal oxide by a mixed liquid of water for hydrizing
the metal alkoxide and alcohol as a solvent having compatibility to
the metal alkoxide and water (hereinafter, the solution is referred
to as a metal alkoxide solution) is coated on a substrate, and the
solution is heated and baked to thereby form a metal oxide film by
a sol-gel method.
In the fourth method, a sol obtained by dispersing fine particles
of a metal oxide as colloid particles in water or an organic
solvent (hereinafter, referred to as a metal oxide sol) is coated
on a substrate. The sol is heated and dried to thereby form a metal
oxide film by a sol-gel method.
The glass frit used for the first method ma be the glass frit
contained in the ceramic color print 14 and the bus bar 15. It is
desirable that the melting point of the glass frit is less than a
temperature suitable to bending glass (600.degree.-640.degree. C.)
because it is necessary for the glass frit to have the melting
point lower than a temperature of baking in order to form a surface
which is sufficiently smooth by the baking operation.
The printing of the glass frit is conducted by a screen-printing
method in the same manner as the formation of the ceramic color
print or the bus bar print, and all printed layers are baked
together as a preferred method.
It is preferable that the glass frit is mixed with a thickening
agent and another additive if required; the viscosity is adjusted
to have a viscosity such as 1,000-20,000 cp (centipoise) desired
for screen printing, and a screen-printing method is employed.
Since the main component of the layer obtained by melting the glass
frit is silicon oxide, the refractive index is about 1.5 and
therefore, it is preferably used as the transparent protective
layer of the present invention.
In the second-fourth methods, a screen printing method, a spray
method, a roll coater method, a meniscus coater method, a print
method, a brushing method and so on can be used to coat the metal
salt solution, the metal alkoxide solution, or the metal oxide sol
(including a material capable of being converted into metal oxide
by baking).
Among methods using the liquid described in the second through
fourth methods, the third method wherein the transparent protective
layer is formed by baking the liquid including the metal alkoxide
is most effective because the liquid can be coated by a simple
screen-printing method. In this case, it is sufficient to adjust
the metal alkoxide, if necessary, in order to obtain a desired
metal oxide. And then, the transparent protective layer 16 of the
present invention is preferably formed by screen-printing a liquid
including a desired metal alkoxide and a thickening agent to adjust
the viscosity suitable for the screen-printing, such as 1,000-2,000
cp (centipoise), followed by baking the liquid.
Further, since one edge portion of the transparent protective layer
16 is received in the gradation print of the ceramic color print,
the protective layer 16 is not conspicuous in its outer appearance
and therefore, there is no strange feeling.
Further, since the particle diameter of the pigment particles in
the ceramic color print 14 after the baking is about 1 .mu.m, it is
necessary to completely cover the uneven surface of the ceramic
color print 14 to make the surface smooth and not to be conspicuous
in appearance. Accordingly, the present invention is to form the
transparent protective layer 16 to have a thickness of 5 m or less,
preferably 1 .mu.m or less.
The transparent protective layer 16 as described above is firmly
attached to the substrate by baking and makes the portion
underlying the transparent conductive layer 13 smooth and reduces
the uneven surface, the underlying portion including the boundary
between the ceramic color 14 and the glass substrate, the surface
of the ceramic color print 14, the boundary between the bus bar 15
and the ceramic color print 14 and a part of the bus bar.
Accordingly, there is no rise in resistance in a local area even by
coating the transparent conductive layer 13 having a thickness far
smaller than the thickness of the ceramic color print 14, and the
breaking of layer can be prevented.
Preferred embodiments of the present invention will be described
with reference to the drawings.
FIG. 1 is an enlarged longitudinal cross-sectional view partly
broken of an embodiment of the electrically heatable windshield of
the present invention.
The electrically heatable windshield comprises an outer glass plate
10 and an inner glass plate 12 bonded to each other through an
interlayer 11 made of a material such as polyvinylbutyral.
A transparent conductive layer 13 as an area heating element is
formed on the contacting surface of one of the glass plates (i.e.
the contacting surface of the outer glass plate 10 in FIG. 1), and
a bus bar 15 for supplying power is laminated on a ceramic color
print 14 for shielding without extending from the color print 14.
Thus, the surface area of the electrically heatable windshield is
heated by supplying power to the transparent conductive layer 13
through the bus bar.
With respect to the boundary insertion of the transparent
protective layer which is the most distinguishable feature of the
present invention, it covers the end portion of the ceramic color
print 14 in the vicinity of the center of the windshield, a portion
of the ceramic color print 14 as an undercoat of the transparent
conductive layer 13, the boundary portion between the bus bar 15
and the ceramic color print 14 and a portion of the bus bar 15 as
an undercoat of the transparent conductive layer 13 to thereby make
the surface portions smooth, whereby increase in resistance at a
local portion which is a cause of the breaking of layer is
prevented and the durability to a current is improved. For easily
understanding, FIG. 2 is a plane view of the outer glass plate and
FIG. 3 is a cross-sectional view taken along a line A--A' in FIG.
2.
As shown in FIG. 3, it is desirable that a taper portion is formed
at the ceramic color print at a portion near the print edge 19 of
the ceramic color print 14 and the transparent protective layer 16
is formed to cover the portion of the ceramic color print.
Further, in order to effectively prevent the breaking of the
transparent conductive layer, it is preferable that an edge portion
20 of the ceramic color print 14 at the surface portion thereof
which faces the transparent protective layer 16 and the transparent
conductive layer and in the vicinity of the center of the
windshield, is tapered to thereby reduce a step.
For the bus bar 15 used for the present invention, silver paste
containing silver and glass frit is preferably used. However,
another material may be used as far as a sufficient effect can be
obtained for the present invention.
On the other hand, for the ceramic color print 14, it may be formed
by printing an ink which is obtained by formulating glass frit, a
pigment such as CuO--Cr.sub.2 O.sub.3, TiO.sub.2, Fe.sub.2 O.sub.3,
CoO, Cr.sub.2 O.sub.3 and filler such as alumina, or by another
suitable method.
The most effective method of forming the protective layer is that
it is coated by a screen-printing method in the same manner as that
for the ceramic color print or the bus bar print and the coated
layers are baked all together at the time of baking.
In order to determine the thickness of the protective layer print
and in a case that the particle diameter of the pigment as a
component of the ceramic color print is generally in a range of
5-15 .mu.m and the protective layer contains pigment particles
having a diameter of about 1 .mu.m after the baking, it is
desirable that the thickness of layer after the calcination is in a
range of 0.01-5 .mu.m, especially 1 .mu.m or less to eliminate
uneven surface. When the thickness is larger than 5 .mu.m, the end
portion of the protective layer becomes conspicuous which is not
desired in its outer appearance. On the other hand, when the
thickness is less than 0.01 .mu.m, the uneven surface may not be
sufficiently eliminated. Accordingly, the thickness should be 0.01
.mu.m or more.
It is necessary to form the ceramic color print because it is
important to conceal the bus bar laminated thereon and to improve
the outer appearance. However, it is necessary to make the surface
of the print smooth in order to improve the performance of electric
conduction. It can be said that the thickness of the printed
transparent protective layer is most desirable from the viewpoint
of the above-mentioned.
For the reasons described above, the width of the printed
transparent protective layer is preferably determined so that the
protective layer partly enter from the edge portion 18 of the bus
bar toward the edge portion of the glass plate 17 by 1-3 mm in the
direction of the glass plane and it partly enters from the edge
portion 19 of the ceramic color print toward the edge portion of
the glass plate by 1-3 mm in the direction of the glass
surface.
Although a material for the transparent conductive layer is not
particularly limited, a construction that a metal oxide such as
SnO.sub.2, ITO, ZnO, TiO.sub.2 or the like is attached to each side
surface of a metal such as Ag or Au is generally used. The sheet
resistance value of the transparent conductive layer is preferably
10 .OMEGA. or less. The thickness of the transparent conductive
layer may be adjusted depending on the purpose of use because it
depends on color tone in its outer appearance and values in
spectroscopy (T.sub.V, T.sub.E, R.sub.V, R.sub.E) (generally, the
thickness in total is in a range of 500-1,000 .ANG.).
The transparent conductive layer 13 is generally attached to the
contacting surface of one of the glass plates. It is preferable to
form the contacting surface of the interlayer 11 facing the inner
side of the outer glass plate 10 so that ice or snow depositing on
the outer surface of the outer glass plate 10 can be easily molten.
As a method of forming the transparent conductive layer, a vacuum
deposition method, a sputtering method or another suitable method
may be used.
In addition to the above-mentioned, an excellent electrically
heatable windshield wherein the transparent protective layer is not
conspicuous can be provided by receiving the end portion (print
edge) of the transparent protective layer in the area of the
gradation print of the ceramic color print.
FIG. 5 shows an example that the transparent protective layer is
received in the ceramic color print, and FIG. 6 is a
cross-sectional view taken along a line C--C' in FIG. 5. In FIGS. 5
and 6, a numeral 14 designates a colored layer such as a ceramic
color print, and a numeral 24 designates a gradation print of such
colored layer.
FIG. 5 shows an example in which the gradation print 24 is formed
in a form of dots. However, another suitable form may be used so
long as the transparent protective layer looks unclearly in an area
extending from the ceramic color print edge 19 in the direction of
the glass surface as a whole.
The transparent protective layer 16 of the present invention covers
the print edge 19 of the colored layer and to prevent the
transparent conductive layer 13 from breaking. Since conductivity
can be sufficiently assured at a portion of the transparent
conductive layer 13 which is directly covered by the glass plates
and in the vicinity of the gradation print, there is small
possibility that the breaking of the layer occurs at the edge
portion of the gradation print 24. Since the print edge 20 of the
transparent protective layer 16 is in the gradation print 24, the
transparent protective layer is not conspicuous in its outer
appearance. The present invention having the above-mentioned
construction can completely eliminate the breaking of layer as has
happened in a conventional windshield.
EXAMPLE 1
A ceramic color print was formed by screen-printing at the
peripheral portion of an outer glass plate 50 as shown in FIG. 2,
followed by predrying it. Silver paste was coated by
screen-printing to form a bus bar as shown in FIG. 2, followed by
predrying it. Glass frit was mixed with a thickening agent and the
mixture was coated by screen-printing as shown in FIG. 4, followed
by drying. Thus formed layers on the outer glass plate were baked
at 630.degree. C. An inner glass plate was overlayed on the outer
glass plate 50 and a bending operation was conducted. The thickness
of the ceramic color print, the thickness of the bus bar, and the
thickness of the transparent protective layer on which the glass
frit was melt-bonded after the baking were respectively 15 .mu.m,
20 .mu.m and about 1 .mu.m. Then, a transparent conductive layer
having a structure of ZnO/Ag/ZnO was formed on the outer glass
plate, and it was bonded to the inner glass plate through an
interlayer to thereby form a laminated glass as shown in FIG. 4.
The terminal resistance between the bus bars of the laminated glass
was 9.53 .OMEGA.. The laminated glass was kept at 80.degree. C. for
30 days. The resistance of the laminated glass was measured to find
9.26 .OMEGA. and there was found neither the breaking of layer nor
uneven current conduction.
EXAMPLE 2
A laminated glass was prepared in the same manner as in Example 1
except that a transparent protective layer is formed by
screen-printing a sol-gel reaction liquid obtained by diluting
silicon alkoxide by alcohol and containing a small amount of water
to form a ceramic color print and by baking simultaneously the
ceramic color print and the bus bar. The thickness of the
transparent protective layer after the baking was 0.1 .mu.m. The
resistance between terminals was 11.31 .OMEGA.. The laminated glass
was kept at 80.degree. C. for 30 days. The resistance of the
laminated glass was 11.54 .OMEGA. and there was found neither the
breaking of layer nor uneven electric conduction.
COMPARATIVE EXAMPLE
A laminated glass as shown in FIG. 10 was prepared in the same
manner as in Example 1 except that no transparent protective layer
was formed. The initial resistance between terminals was 9.53
.OMEGA.. The laminated glass was kept at 80.degree. C. for 30 days.
The resistance after that was 11.63 .OMEGA.. When a current was
supplied to the laminated glass, the breaking of layer
occurred.
The electrically heatable windshield of the present invention
performs the following effects.
Since a transparent protective layer covers a ceramic color print
and a bus bar constituting the undercoat of a transparent
conductive layer, especially an end portion of the ceramic color
print near the center of a glass plate, occurrence of the breaking
of layer and uneven current conduction are suppressed and increase
of the sheet resistance of the transparent conductive layer at this
portion can be prevented. Accordingly, increase in the resistance
at a local portion is avoidable, hence the breaking of layer can be
prevented even though a transparent conductive layer having a
thickness extremely smaller than thickness of the ceramic color
print is coated.
Further, by forming the transparent protective layer 16 by a
printing method at the boundary between the bus bar 15 and the
ceramic color print 14, it is very effective to prevent the
breaking of layer and improvement in durability to current
conduction.
Since a print edge of the transparent protective layer 16 apart
from the bus bar is received in the gradation print of the ceramic
color print 14, an electrically heatable windshield having good
outer appearance can be provided.
Further, since the refractive index of the transparent protective
layer is 2.0 or less, preferably in a range of 1.4-1.6 and it is
close to the refractive index of glass which is used as a
substrate, occurrence of optical interference can be reduced and an
electrically heatable windshield having good outer appearance can
be provided.
In addition, since the thickness of the transparent protective
layer is determined to be in a range of 0.01-5 .mu.m, particularly
1 .mu.m or less, the portions boundary between the bus bar and the
ceramic color print, the surface of the ceramic color print, the
boundary between the ceramic color print and the glass plate and an
undercoat portion of the transparent conductive layer are made
smooth.
* * * * *